The orderly transport of proteins within the secretory pathway of eucaryotic cells is mediated by the specific recognition and fusion of transport vesicles with distinct target organelle-associated membrane proteins termed SNAREs. In our attempt to understand protein trafficking in lymphocytes, we have cloned novel SNAREs or SNARE- related proteins. Using the yeast two-hybrid system, we have isolated the cDNA for SNAP-23, a ubiquitously expressed membrane-associated protein involved in facilitating vesicle docking and fusion with the plasma membrane. We have also identified the novel kinase SNAK that phosphorylates SNAP-23 and regulates its association with syntaxin, another membrane trafficking protein. In addition, we have identified a novel syntaxin that localizes to late endosomes and are investigating the function of this protein in lymphocyte biology.We are also investigating the role of post-translational modifications in the transport of MHC class II molecules to antigen processing compartments. We have identified the phosphorylation sites on the MHC class II- associated invariant chain, have identifed protein kinase C as the kinase responsible for invariant chain phosphorylation, and are investigating the possibility that invariant chain phosphorylation enhances antigen presentation during the immune response. - antigen presentation, B lymphocytes, major histocompatibility complex class II, protein phosphorylation, protein transport,